Insulin-like growth factor-I receptor (IGF-I receptor) is a transmembrane heterotetrameric protein, which has two extracellular alpha chains and two membrane-spanning beta chains in a disulfide-linked β-α-α-β configuration. The binding of the ligands, which are insulin-like growth-factor-I (IGF-I) and insulin-like growth factor-II (IGF-II), by the extracellular domain of IGF-I receptor activates its intracellular tyrosine kinase domain resulting in autophosphorylation of the receptor and substrate phosphorylation. The IGF-I receptor is homologous to insulin receptor, having a high sequence similarity of 84% in the beta chain tyrosine kinase domain and a low sequence similarity of 48% in the alpha chain extracellular cysteine rich domain (Ulrich, A. et al., 1986, EMBO, 5, 2503-2512; Fujita-Yamaguchi, Y. et al., 1986, J. Biol. Chem., 261, 16727-16731; LeRoith, D. et al., 1995, Endocrine Reviews, 16, 143-163). The IGF-I receptor and its ligands (IGF-I and IGF-II) play important roles in numerous physiological processes including growth and development during embryogenesis, metabolism, cellular proliferation and cell differentiation in adults (LeRoith, D., 2000, Endocrinology, 141, 1287-1288; LeRoith, D., 1997, New England J. Med., 336, 633-640).
IGF-I and IGF-II function both as endocrine hormones in the blood, where they are predominantly present in complexes with IGF-binding proteins, and as paracrine and autocrine growth factors that are produced locally (Humbel, R. E., 1990, Eur. J. Biochem., 190, 445-462; Cohick, W. S. and Clemmons, D. R., 1993, Annu. Rev. Physiol. 55, 131-153).
The IGF-I receptor has been implicated in promoting growth, transformation and survival of tumor cells (Baserga, R. et al., 1997, Biochem. Biophys. Acta, 1332, F105-F126; Blakesley, V. A. et al., 1997, Journal of Endocrinology, 152, 339-344; Kaleko, M., Rutter, W. J., and Miller, A. D. 1990, Mol. Cell. Biol., 10, 464-473). Thus, several types of tumors are known to express higher than normal levels of IGF-I receptor, including breast cancer, colon cancer, ovarian carcinoma, synovial sarcoma and pancreatic cancer (Khandwala, H. M. et al., 2000, Endocrine Reviews, 21, 215-244; Werner, H. and LeRoith, D., 1996, Adv. Cancer Res., 68, 183-223; Happerfield, L. C. et al., 1997, J. Pathol., 183, 412-417; Frier, S. et al., 1999, Gut, 44, 704-708; van Dam, P. A. et al., 1994, J. Clin. Pathol., 47, 914-919; Xie, Y. et al., 1999, Cancer Res., 59, 3588-3591; Bergmann, U. et al., 1995, Cancer Res., 55, 2007-2011). In vitro, IGF-I and IGF-II have been shown to be potent mitogens for several human tumor cell lines such as lung cancer, breast cancer, colon cancer, osteosarcoma and cervical cancer (Ankrapp, D. P. and Bevan, D. R., 1993, Cancer Res., 53, 3399-3404; Cullen, K. J., 1990, Cancer Res., 50, 48-53; Hermanto, U. et al., 2000, Cell Growth & Differentiation, 11, 655-664; Guo, Y. S. et al., 1995, J. Am. Coll. Surg., 181, 145-154; Kappel, C. C. et al., 1994, Cancer Res., 54, 2803-2807; Steller, M. A. et al., 1996, Cancer Res., 56, 1761-1765). Several of these tumors and tumor cell lines also express high levels of IGF-I or IGF-II, which may stimulate their growth in an autocrine or paracrine manner (Quinn, K. A. et al., 1996, J. Biol. Chem., 271, 11477-11483).
Epidemiological studies have shown a correlation of elevated plasma level of IGF-I (and lower level of IGF-binding protein-3) with increased risk for prostate cancer, colon cancer, lung cancer and breast cancer (Chan, J. M. et al., 1998, Science, 279, 563-566; Wolk, A. et al., 1998, J. Natl. Cancer Inst., 90, 911-915; Ma, J. et al., 1999, J. Natl. Cancer Inst., 91, 620-625; Yu, H. et al., 1999, J. Natl. Cancer Inst., 91, 151-156; Hankinson, S. E. et al., 1998, Lancet, 351, 1393-1396). Strategies to lower the IGF-I level in plasma or to inhibit the function of IGF-I receptor have been suggested for cancer prevention (Wu, Y. et al., 2002, Cancer Res., 62, 1030-1035; Grimberg, A and Cohen P., 2000, J. Cell. Physiol., 183, 1-9).
The IGF-I receptor protects tumor cells from apoptosis caused by growth factor deprivation, anchorage independence or cytotoxic drug treatment (Navarro, M. and Baserga, R., 2001, Endocrinology, 142, 1073-1081; Baserga, R. et al., 1997, Biochem. Biophys. Acta, 1332, F105-F126). The domains of IGF-I receptor that are critical for its mitogenic, transforming and anti-apoptotic activities have been identified by mutational analysis.
For example, the tyrosine 1251 residue of IGF-I receptor has been identified as critical for anti-apoptotic and transformation activities but not for its mitogenic activity (O'Connor, R. et al., 1997, Mol. Cell. Biol., 17, 427-435; Miura, M. et al., 1995, J. Biol. Chem., 270, 22639-22644). The intracellular signaling pathway of ligand-activated IGF-I receptor involves phosphorylation of tyrosine residues of insulin receptor substrates (IRS-1 and IRS-2), which recruit phosphatidylinositol-3-kinase (PI-3-kinase) to the membrane. The membrane-bound phospholipid products of PI-3-kinase activate a serine/threonine kinase Akt, whose substrates include the pro-apoptotic protein BAD which is phosphorylated to an inactive state (Datta, S. R., Brunet, A. and Greenberg, M. E., 1999, Genes & Development, 13, 2905-2927; Kulik, G., Klippel, A. and Weber, M. J., 1997, Mol. Cell. Biol. 17, 1595-1606). The mitogenic signaling of IGF-I receptor in MCF-7 human breast cancer cells requires PI-3-kinase and is independent of mitogen-activated protein kinase, whereas the survival signaling in differentiated rat pheochromocytoma PC12 cells requires both PI-3-kinase and mitogen-activated protein kinase pathways (Dufourny, B. et al., 1997, J. Biol. Chem., 272, 31163-31171; Parrizas, M., Saltiel, A. R. and LeRoith, D., 1997, J. Biol. Chem., 272, 154-161).
Down-regulation of IGF-I receptor level by anti-sense strategies has been shown to reduce the tumorigenicity of several tumor cell lines in vivo and in vitro, such as melanoma, lung carcinoma, ovarian cancer, glioblastoma, neuroblastoma and rhabdomyosarcoma (Resnicoff, M. et al., 1994, Cancer Res., 54, 4848-4850; Lee, C.-T. et al., 1996, Cancer Res., 56, 3038-3041; Muller, M. et al., 1998, Int. J Cancer, 77, 567-571; Trojan, J. et al., 1993, Science, 259, 94-97; Liu, X. et al., 1998, Cancer Res., 58, 5432-5438; Shapiro, D. N. et al., 1994, J. Clin. Invest., 94, 1235-1242). Likewise, a dominant negative mutant of IGF-I receptor has been reported to reduce the tumorigenicity in vivo and growth in vitro of transformed Rat-1 cells overexpressing IGF-I receptor (Prager, D. et al., 1994, Proc. Natl. Acad. Sci. USA, 91, 2181-2185).
Tumor cells expressing an antisense to the IGF-I receptor mRNA undergo massive apoptosis when injected into animals in biodiffusion chambers. This observation makes the IGF-I receptor an attractive therapeutic target, based upon the hypothesis that tumor cells are more susceptible than normal cells to apoptosis by inhibition of IGF-I receptor (Resnicoff, M. et al., 1995, Cancer Res., 55, 2463-2469; Baserga, R., 1995, Cancer Res., 55, 249-252).
Another strategy to inhibit the function of IGF-I receptor in tumor cells has been to use anti-IGF-I receptor antibodies which bind to the extracellular domains of IGF-I receptor and inhibit its activation. Several attempts have been reported to develop mouse monoclonal antibodies against IGF-I receptor, of which two inhibitory antibodies—IR3 and 1H7—are available and their use has been reported in several IGF-I receptor studies.
The IR3 antibody was developed using a partially purified placental preparation of insulin receptor to immunize mice, which yielded an antibody, IR1, that was selective for binding insulin receptor, and two antibodies, IR2 and IR3, that showed preferential immunoprecipitation of IGF-I receptor (somatomedin-C receptor) but also weak immunoprecipitation of insulin receptor (Kull, F. C. et al., 1983, J. Biol. Chem., 258, 6561-6566).
The 1H7 antibody was developed by immunizing mice with purified placental preparation of IGF-I receptor, which yielded the inhibitory antibody 1H7 in addition to three stimulatory antibodies (Li, S.-L. et al., 1993, Biochem. Biophys. Res. Commun., 196, 92-98; Xiong, L. et al., 1992, Proc. Natl. Acad. Sci. USA, 89, 5356-5360).
In another report, a panel of mouse monoclonal antibodies specific for human IGF-I receptor were obtained by immunization of mice with transfected 3T3 cells expressing high levels of IGF-I receptor, which were categorized into seven groups by binding competition studies and by their inhibition or stimulation of IGF-I binding to transfected 3T3 cells (Soos, M. A. et al., 1992, J. Biol. Chem., 267, 12955-12963).
Thus, although IR3 antibody is the most commonly used inhibitory antibody for IGF-I receptor studies in vitro, it suffers from the drawback that it exhibits agonistic activity in transfected 3T3 and CHO cells expressing human IGF-I receptor (Kato, H. et al., 1993, J. Biol. Chem., 268, 2655-2661; Steele-Perkins, G. and Roth, R. A., 1990, Biochem. Biophys. Res. Commun., 171, 1244-1251). Similarly, among the panel of antibodies developed by Soos et al., the most inhibitory antibodies 24-57 and 24-60 also showed agonistic activities in the transfected 3T3 cells (Soos, M. A. et al., 1992, J. Biol. Chem., 267, 12955-12963). Although, IR3 antibody is reported to inhibit the binding of IGF-I (but not IGF-II) to expressed receptors in intact cells and after solubilization, it is shown to inhibit the ability of both IGF-I and IGF-II to stimulate DNA synthesis in cells in vitro (Steele-Perkins, G. and Roth, R. A., 1990, Biochem. Biophys. Res. Commun., 171, 1244-1251). The binding epitope of IR3 antibody has been inferred from chimeric insulin-IGF-I receptor constructs to be the 223-274 region of IGF-I receptor (Gustafson, T. A. and Rutter, W. J., 1990, J. Biol. Chem., 265, 18663-18667; Soos, M. A. et al., 1992, J. Biol. Chem., 267, 12955-12963).
The MCF-7 human breast cancer cell line is typically used as a model cell line to demonstrate the growth response of IGF-I and IGF-II in vitro (Dufourny, B. et al., 1997, J. Biol. Chem., 272, 31163-31171). In MCF-7 cells, the IR3 antibody incompletely blocks the stimulatory effect of exogenously added IGF-I and IGF-II in serum-free conditions by approximately 80%. Also, the IR3 antibody does not significantly inhibit (less than 25%) the growth of MCF-7 cells in 10% serum (Cullen, K. J. et al., 1990, Cancer Res., 50, 48-53). This weak inhibition of serum-stimulated growth of MCF-7 cells by IR3 antibody in vitro may be related to the results of an in vivo study in which IR3 antibody treatment did not significantly inhibit the growth of a MCF-7 xenograft in nude mice (Arteaga, C. L. et al., 1989, J. Clin. Invest., 84, 1418-1423).
Because of the weak agonistic activities of the IR3 and other reported antibodies, and their inability to significantly inhibit the growth of tumor cells such as MCF-7 cells in the more physiological condition of serum-stimulation (instead of stimulation by exogenously added IGF-I or IGF-II in serum-free condition), there is a need for new anti-IGF-I receptor antibodies which significantly inhibit the serum-stimulated growth of tumor cells but which do not show significant agonistic activity by themselves.